Nested bodies need to have pre-defined opaques

[compiler-errors]

https://rust.godbolt.org/z/fjjTxrYnh

#![feature(type_alias_impl_trait)]

struct Foo;
trait Bar {}
impl Bar for Foo {}

type Tait<'a> = impl Sized;

fn needs_bar<'a>() where Tait<'a>: Bar {
    let x: Tait<'a> = Foo;
}

fn foo<'a>() -> Tait<'a> {
    || {
        needs_bar();
    };
    Foo
}

But they also may not define the opaque, so we have to be ok w/ that too.

This example does not compile with the old solver as-well, however the old solver may have non-defining uses of the opaque in nested bodies, e.g.

pub fn defines_in_closure<'a>() -> impl Sized + 'a {
    let _ = || defines_in_closure::<'a>();
}

[compiler-errors]

Probably need to predefine opaques in the nested bodies, but in order to not require them to also always define those opaques, we need to propagate them to the body owner the same way we do with closure region requirements.